Electronic structure configuration-interaction methods

Atextbook describing the theory associated with calculations of the electronic structure of molecular systems. While the book focuses on ab initio calculations, much of the information is also relevant to semi-empirical methods. The sections on the Hartree-Fock and Configuration Interactions methods, in particular, apply to HyperChem. The self-paced exercises are useful for the beginning computational chemist. 

B.O. Roos and P.E.M. Siegbahn, The Direct Configuration Interaction Method from Molecular Integrals, in Methods of Electronic Structure Theory (H.F. Schaefer III, ed.), Plenum Press, New York (1977). 

Electronic configurations are the MO equivalents of resonance structures. Sometimes a molecular state cannot adequately be represented by a single configuration, just as benzene or an enolate ion cannot be represented by only one Kekule structure. The molecular state is then better described by a linear combination of several electronic configurations (configuration interaction method). 

Other theoretical studies of ozone and its excited states have been previously reported and also indicate a relatively low lying state corresponding to a six jr-electron cyclic structure. In particular, recent ab initio calculations using the generalized valence bond and configuration interaction methods with a double zeta basis set, predict that trioxirane lies about 35 kcal/mol higher in energy than ozone but that it is a potential minimum. 

Several sets of theoretical calculations have been performed on the parent ring system. HMO calculations of total rr-electron densities and frontier electron densities successfully predicted that the nucleus would undergo electrophilic substitution at the 6- and 8-positions. Two groups, " have compared the electronic structure of indolizine and various aza derivatives using the SCF or semiempirical antisymmetric configuration interaction method. The results allowed interpretations of the electronic spectrum to be made which were in good agreement with experiment. 

For a recent review of configuration interaction methods, see I. Shavitt, H. F. Schaeffer 111, Ed., in Modern Theoretical Chemistry, Vol. Ill, Methods of Electronic Structure Theory, Plenum Press, New York, 1976. 

Relativistic and electron correlation effects play an important role in the electronic structure of molecules containing heavy elements (main group elements, transition metals, lanthanide and actinide complexes). It is therefore mandatory to account for them in quantum mechanical methods used in theoretical chemistry, when investigating for instance the properties of heavy atoms and molecules in their excited electronic states. In this chapter we introduce the present state-of-the-art ab initio spin-orbit configuration interaction methods for relativistic electronic structure calculations. These include the various types of relativistic effective core potentials in the scalar relativistic approximation, and several methods to treat electron correlation effects and spin-orbit coupling. We discuss a selection of recent applications on the spectroscopy of gas-phase molecules and on embedded molecules in a crystal enviromnent to outline the degree of maturity of quantum chemistry methods. This also illustrates the necessity for a strong interplay between theory and experiment. 

However, from a computational point of view, the LDA is much easier to implement and, relative to configuration interaction methods, is much less computationally intensive. Yin and Cohen, along with others, had used the LDA to construct pseudopotentials from first principles [5]. These ab initio pseudopotentials were very helpful in describing the electronic structure of surfaces, molecules, defects, etc., but they were not viewed as tools for computing the total energy of a solid. 

Roos, B. O. and Siegbahn, P. E. M., The direct configuration interaction method firom molecular integrals, in Methods of Electronic Structure Theory H. F. Schaefer HI (Ed.), Plenum, New York, 1977, p. 277. A discussion by the originators of an important metht in which the Cl matrix is never explicitly constructed. 

Most popular in the ab initio calculation of intermolecular potentials is the so-called supermolecule method, because it allows the use of standard computer programs for electronic structure calculations. This method automatically includes all the electrostatic, penetration and exchange effects. If the calculations are performed at the SCF (self-consistent field) level the induction effects are included, too, but the dispersion energy is not. The latter, which is an intermolecular electron correlation effect, can be obtained by configuration interaction (Cl), coupled cluster (CC) calculations or many-body perturbation theory (MBPT). These calculations are all plagued... 

As in the Hartree-Fock molecular orbital theory, which is based on the independent particle model, the above Hartree product method also lacks enough correlation among the orbitals, and thereby the resultant accuracy is limited. To overcome the drawback, one can take account of the interaction among possible configurations (or the Hartree products) as in the configuration interaction method and multiconfiguration SCF methods in electronic structure theory. The multiconfigulational time-dependent Hartree... 

Configuration Interaction Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field Linear Scaling Methods for Electronic Structure Calculations Pseudospectral Methods in Ab Initio Quantum Chemistry. 

The quantum theory of the electronic structure of infinite, periodic macromolecules is a well-developed and venerable topic which is reviewed often (1,12) and is well-known to the audience of this NATO school. The important aspect of this topic for our present purposes is the observation that ab initio (i.e., Hartree Fock plus configuration interaction) methods are simply too expensive to apply to the description of most of the large molecules (including polymers) of biological and technological interest. In addition, they fail to provide a quantitative description of UV absorption spectra without massive configuration-interaction analysis. Consequently, for quantum chemical calculations to be helpful in the design of polymeric materials of practical use in electronic applications (e.g., photoconductors (2,5), semiconductors (5), or battery electrodes (6)), some form of semiempirical model must be developed. 

H. Moriyama, H. Tatewaki, and S. Yamamoto, Electronic structure of CeO studied by a four-component relativistic configuration interaction method, J. Chem. Phys. 138, 224310 (8 pages) (2013). 

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